In the capillary transport of liquids between opposing surfaces, two liquids can be brought together by flowing in opposing directions, creating a flow in opposition, or they can be transported in a concurrent flow wherein they advance simultaneously and together through the same part of the zone. In the first case, the intent can be to have only one of the two liquids in any one of two parts of the zone, the liquids meeting at a junction between the two parts. In the second, concurrent flow case, the intent can be for each of the liquids to traverse essentially all of the transport zone, arriving in generally equal amounts at a final destination.
In either case, it can be important that the liquids flow in a controlled manner. For example, if opposing flow transport is being used as an ion bridge between ion-selective electrodes, hereinafter "ISE", two liquids are introduced into the spacing between the surfaces to advance in opposite directions ideally at equal rates to meet at a predetermined junction, as explained, for example, in my U.S. Pat. No. 4,271,119, issued on June 2, 1981. However, when testing biological liquids against a reference liquid having a different viscosity and/or surface tension, using the differential analysis of the aforesaid patent, it is common for the one liquid to flow much faster than the other. If the faster flow pushes into contact with the ISE that is intended for the other liquid, the test is ruined. To keep this from happening, I have disclosed techniques such as the coating of at least one of the opposing capillary surfaces with a water-swellable or water-dissolvable substance, as described, for example, in my U.S. application Ser. No. 537,553, filed on Oct. 3, 1983, now U.S. Pat. No. 4,549,952, entitled "Capillary Transport Device Having Means for Increasing the Viscosity of the Transported Liquid", which is a continuation-in-part application of U.S. Ser. No. 443,785, filed on Nov. 22, 1982 now abandoned. Although such coatings are very effective, they do require the additional step of applying the coating. In some cases it would be advantageous if a speed-of-flow control could be constructed that does not require an additional layer of material. On the other hand, mechanical constraints to flow tend to be objectionable because they can cause air entrapment. Such air entrapment is undesirable as it tends to unpredictably interfere with flow through the transport. A capillary transport device is described in my U.S. Pat. No. 4,233,029, issued on Nov. 11, 1980, having ribs that restrain the flow between capillary surfaces while avoiding air entrapment. However, to make the flow completely predictable, the ribs are provided on both of the opposed capillary surfaces. It is desirable at least from the standpoint of production to provide controlled flow wherein at least one of the opposing capillary surfaces is left generally smooth. Prior to this invention, it has not been clear how this could be done and still avoid air entrapment.